I hear our closest living relatives, chimpanzees and bonobos have lots of sexual partners. That got me wondering if we are naturally monogamous or if it's a social construction.

(Answering purely as a scientist and leaving any ethical/religious convictions out of it ...)

What is "natural"?  Isn't society natural?  And if so, isn't a social construction natural?  I think this question presents a false dichotomy.

Hi Joon, this is an interesting question and although there are no definative answers we can make some observations that give us an idea about this.

Monogamous animals (e.g. gibbons) tend to have males and females of the same size and general appearance. Human males are on average 10% bigger than females. Males also have more facial and body hair, broader shoulders and more muscle than females, females have wider hips and breasts. This sexual dimorphism suggests that humans have not evolved to be entirely monogamous. However, the differences in size etc. are not as great as the differences seen between males and females of habitually polygamous species.

Sexual behaviour tends to depend on the environment to quite a large extent. If an environment is resource poor then it can be advantageous to form a monogamous pair, driving off intruders, to improve the likelihood of survival for your young. This sort of behaviour has been recorded in lions living in the Namibian desert, even though the species is highly polygamous when in a more resource-rich habitat.

Human cultures have been heavily influenced by religion for a very long time. Some religions demand monogamy, and it is interesting that these religions have their roots in resource-poor environments. It seems plausible that the religious basis to our society has created pressure to be monogamous (e.g. laws against bigamy) in an environment that would otherwise favour polygamy, polyandry or even promiscuity. It is unsurprising that sexual relationships can be very complicated in the modern world.

It should be noted that human offspring are highly altricial, so it takes a long time for them to become independent. This favours lasting partnerships, since the offspring have a better chance of survival with more than one adult caring for them. However, this does not require monogamy, since a group is even better than a pair at providing care.

Following on from Paolo's comments, it's interesting to note that other societies elsewhere in the world have very different systems. In Tibetan culture, it is considered normal for several husbands to share a single wife, while in many other regions polygamy (several wives for one husband) is preferred. Whether this is because of local resource conditions or simple cultural variation is a topic of great debate among anthropologists.

You are right to note that our closest ancestors are not monogamous, and even those related species that are (e.g. gorillas) have physical differences that suggest we are naturally different. That's not to say, however, that our species might not be changing - perhaps we are evolving to be monogamous?